Geology Reference
In-Depth Information
It may not be possible to carry a large polariscope eve-
rywhere to the field. For these situations a simple method
can be adapted for quick‐and‐easy evaluation of ice
types, if necessary, using 35 mm SLR digital cameras.
Thin sections are made using what may be called “the
palm technique.” The procedure can be extremely simple.
It works well if the ice to be examined is thin. Otherwise
a thick section is cut with a handheld saw such as a hack-
saw to about 5 mm thick. The section thickness is then
reduced by successive melting from both sides using
palms (if no other alternatives are available) to a thick-
ness of about 1 mm. The thinning process should be pro-
gressive with periods of cooling. Reasonably good thin
sections can be made with some practice, no matter how
ridiculous the procedure may sound.
The handheld polariscope consists of SLR camera and
two linearly polarizing filters, as mentioned earlier, which
can be purchased from most camera shops. The filter
mounted on the camera can act as the analyzer, whereas
the other filter can be used as the polarizer. In case the
second polarizing filter (which can be used as the polar-
izer) is not available, some possible options are available.
The best option is to use the empty white screen [liquid‐
crystal display (LCD) monitor] of a laptop or other
devices that use such monitors, as the source of linearly
polarized light. LCD monitors provide a uniformly lit
source of linearly polarized light with axis of polariza-
tion usually at 45° to the horizontal. The intensity of
most laptop screens can also be varied to some extent.
Use manual focusing and aperture/time adjustments in
case the autofocus and light‐metering system of the digi-
tal SLR does not work.
Since reflected light, as has already been discussed in
section 6.1.1., is elliptically polarized with major axis in
the plane of reflection, light from any source after reflec-
tion from a flat nonmetallic surface such as glass or plas-
tic can be used as “a poor polarizer.” For a horizontally
oriented glass plate, an incident angle of about 56° from
the vertical (the Brewster angle) is the best. Another
option is to use the sky light. In the Arctic or the Antarctic,
particularly during the clear polar days, the light from
blue sky at 90° to the direction of the sun can be used as
a poor polarizer because scattered light from the sky is
strongly polarized.
The palm technique for making thin sections in con-
junction with light from a vehicle, two polarizing fil-
ters, and a 35 mm digital SLR camera was actually used
by Sinha [2004] for examining the type of ice on a run-
way  at Erding Airbase in Germany during a series of
friction tests under the Joint Winter Runway Friction
Measurement Program (JWRFMP). This was an interna-
tional collaborative project to conduct field tests using
different types of instrumented aircraft and ground
friction measuring vehicles. As such, microstructural
analysis of the winter contaminants on the runway was
not part of the original plan and consequently neces-
sary  equipment were not taken to the field. JWRFMP
was a program between Transport Canada (TC), the
U.S.  National Aeronautics and Space Administration
(NASA), the U.S. Federal Aviation Administration
(FAA), the National Research Council Canada (NRCC),
the Canadian Department of National Defence (DND),
the Norwegian Civil Aviation Administration, and sev-
eral airframe and ground friction measuring equipment
manufacturers.
6.3.2. Cross‐Polarized Versus Parallel‐Polarized
Light Viewing
Too much emphasis is given in the ice literature on the
use of cross‐polarized light to view thin sections. This is
primarily due to the fact that crossed polarizers offer the
highest contrast between colors of crystals with differ-
ent  orientations. However, it should be mentioned that
the  orientation of the analyzer can be rotated to any
polarization direction with respect to the polarizer. One
particular direction is the parallel polarizers. This combi-
nation offers the lowest contrast in interference colors
and consequently does not produce spectacular colors.
For that reason little attention has actually been paid to
the use of parallel‐polarized light for viewing thin sec-
tions of ice. Nonetheless, the parallel polarizers have
unique advantages for the analysis of microstructures of
ice with inclusions. An example of a comparative study
between cross‐polarized and parallel‐polarized images is
given in Figure 6.13 to show the usefulness of the latter
viewing method in conjunction with the former.
In cross‐polarized images, crystals with their c axis
parallel to either of the axis of orientation of the polar-
izer and the analyzer, and/or parallel to the direction of
light propagation, appear as black. These three positions
are known as positions of extinction and are, in fact, the
basis for determining texture and hence fabric diagrams
of ice using universal stages. When a crystal (grain) is
dark or black, the detailed intragranular structures,
such as bubbles or cracks, become invisible. This can be
seen if the dark crystals in the left image of Figure 6.13
are compared with those same crystals in the image on
right. The relatively lighter interference colors in most of
the crystals in the right image reveal the size, shape, and
orientations of air/gas bubbles entrapped in the shelf ice
during the consolidation of snow particles or small intra-
granular (confined within the crystal) and intergranular
(at the grain or crystal boundaries) cracks. Healed cracks
with their planes parallel to either of the two main cleav-
age planes of ice could be detected by the presence of a
row of air bubbles. Most air bubbles were found to be
elongated with their long dimensions parallel to the basal
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